1. Field of the Invention
The present invention relates to methods of forming mechanical joints between lengths of field-grade pipe of the type and quality typically utilized in oil field applications, such as gathering lines for distribution systems down-hole casing, and the like, and to the joints so formed. It is particularly adapted for joining materials heretofore considered highly difficult if not impossible to join mechanically with consistent reliability, namely, plain aluminum pipe, internally-coated pipe, and externally-coated pipe.
2. Description of the Prior Art
The literature contains many schemes for mechanically joining pipes, tubing and the like, only a few of which have proved actually capable of forming leak-free joints with oilfield materials. One such method known to be actually operative is that disclosed in U.S. Pat. No. 3,208,136. Such patent discloses a pipe-joining method consisting essentially of forming a bell on one pipe end, rolling an annular groove in another pipe end, and telescoping the latter into the former. Additionally, one or the other of the two surfaces was typically coated with a liquid lubricant-sealant prior to the joining of the pipes. Related U.S. Pat. No. 3,210,102 discloses the coupling or joint essentially formed by the above-described method.
U.S. Pat. No. 3,466,738 discloses a method of joining pipe consisting essentially of simply forcing together two pipe ends of like diameter, the only preparatory step being the slight flaring of one tip in order to permit the initial entry of the other. The amount of force and the time required to so join two such pipe ends is many times greater than that required by the method of the present invention, and it is likely that galling of the parts can occur.
Enclosed British patent specification No. 435,700 discloses a tubular joining method which is in some respects similar to both that of U.S. Pat. No. 3,466,738 and that of the present invention. FIG. 3 of said specification discloses a slight bell at the mouth of the outer member whereas FIG. 1 discloses a chamfer on the outside edge of the inner member. The two members are to be forced together without the application of heat and in such a manner as to stress at least one member beyond its elastic limit. The compound elastic-plastic strain is achieved by dimensioning the members prior to joining such that the inner diameter of the outside member is only slightly smaller than the outside diameter of the inner member. The provisional portion of said Specification discloses the desirability of, alternatively, forming the portions to be joined as opposed cones or one or both such portions having annular grooves and/or corresponding steps. No drawing of such alternative construction is provided, but it appears that the construction alluded to is of the "snap-in" or "locking" variety. It is noted that all joints illustrated are of cross-sectional area considerably reduced from that of at least one of the tubular members.
U.S. Pat. No. 3,217,400 discloses a method of connecting thermoplastic pipe sections and the like in which such sections containing similar annular bulges are hammered together until the bulge of the inner section engages the inner side of the bulge of the outer section. U.S. Pat. No. 3,828,412 discloses the joining of tapered, thin aluminum tubes, of the type typically used in the refrigeration field, by coating one member with epoxy prior to insertion and subsequently curing said epoxy to form a purely adhesive lap joint. U.S. Pat No. 3,785,025 discloses a method of constructing a hermetic compressor discharge tube joint wherein a male member having two cylindrical seals is partially inserted into the other member, liquid adhesive is poured into the annular well therebetween, insertion is then completed and the adhesive permitted to cure to form a seal gasket. None of these methods is, however, applicable to the joining of elements such as pipe, casing, and the like, and none of the known methods of others is believed operable when applied to such field-grade elements.